GBZ PWM Audio Filter Frustrations!

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aedlin61
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Joined: Thu Oct 14, 2021 4:00 am

GBZ PWM Audio Filter Frustrations!

Post by aedlin61 » Thu Oct 14, 2021 5:00 am

Hi guys,

Currently mid-build for my Game Boy Zero and have run into audio issues...

I am using PAM8403 for my amp and have made a filter to try clean up the sound but the sound doesn't really change with the filter or without. This is the filter i am using https://learn.adafruit.com/assets/35685. I am using it in mono so have only built the top half.

For wiring i have RPI Physical Pin 12 to Filter, Filter to Amp L-IN, L-OUT to Speaker. The Amp is running to 5v supply from RPI. I have tried with and without a pot. The audio i get is a good volume but i have loads of interference whichever way i try to go. Am i just expecting too much? Any advice appreciated!

floppes
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Re: GBZ PWM Audio Filter Frustrations!

Post by floppes » Fri Oct 15, 2021 2:03 pm

Digital logic creates noise on the power supply rail. Take a look at the schematic of the Raspberry Pi 3 and you will see that they have a separate 3.3 V power supply for audio (3V3A) from their PMIC (MxL7704). They use an NC7WZ16 buffer to decouple the audio signal:
Pi audio.png
Pi audio.png (66.14 KiB) Viewed 221 times
You can use an NC7WZ16 together with a low noise voltage regulator for this.

Additionally, use star grounding for your analog signals.

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